setting default applications

[NW]

I have just installed truecrypt on my laptop (Mint v 9) and tested it.
(I have used it before.) I found that, although I could mount the
truecrypt file I could not open it. It was looking for Nautilus, which
is a program I have never used and was not installed. So I installed it
and now it works. 

This made me  wonder, why is it looking for Nautilus and why can't I
tell it which program to use.? I know that I can select which are my
preferred applications but only the browser and the mail client. Is
there any way to set up other preferences please, such as text editor
and file manager?

Neil